This invention relates to a method of manufacturing a heat resisting magnetic scale for use in high temperature regions.
FIG. 1 is a cross-sectional view of a conventional, prior art, magnetic scale disclosed, for example, in Japanese Patent Publication No. 10655/1973. Referring to the figure, a non-magnetic metal layer 7 such as copper or aluminum which is formed by plating or cladding is applied to the surface of a base 6 which is made of steel or steel alloy such as one sold under the tradename "Elinvar" in the form of a bar with a circular cross-sectional shape. The surface of the non-magnetic metal layer 7 has a magnetic layer 8 made of Cu-Ni alloy applied thereto.
Conventional magnetic scales can be used in, for example, precision machine tools when mounted on the same. A magnetic scale of the type described above first records signals (N, S, magnetization) at a predetermined interval on the magnetic layer 8 in the longitudinal direction of the base 6. Next, a magnetic head (omitted from the illustration) is brought into contact with the magnetic layer 8, and this magnetic head and the base 6 on which the signals are recorded are moved relative to each other. As a result of this, the relative position can be detected by the magnetic head.
As shown in Table 1.2.6 and 6.6.4 of the "Metal Data Book" (edited by Japan Metal Society, 1974), the coefficients of thermal expansion of iron and an iron-alloy such as "Elinvar" (tradename) are each 12.1.times.10.sup.-6 and 8.0.times.10.sup.-6. The coefficients of thermal expansion of copper and aluminum are respectively 17.0.times.10.sup.-6 and 23.5.times.10.sup.-6. As also shown in Table 6 (6-2) of "Heat Resisting Steel Data" (edited by Special Steel Club, 1965), the coefficient of thermal expansion of Cu-Ni alloy is, for example, 11.9.times.10.sup.-6 (AISI 21.degree. to 316.degree. C.) on S-816 (AISI No. 671). In a magnetic scale of the type shown in FIG. 1, the coefficient of thermal expansion of the magnetic scale is determined to a substantial extent by the coefficient of thermal expansion of the base 6. However, if a magnetic scale of the type described above is used in a high temperature range of 100.degree.- 300.degree. C., the non-magnetic metal layer 7 or the magnetic layer 8 will inevitably be separated from the base 6 due to the difference in the degree of expansion as between the base 6, the non-magnetic metal layer 7 and the magnetic layer 8. This difference is due to the fact that the base 6, the non-magnetic metal layer 7, and the magnetic layer 8 each have different coefficients of thermal expansion.
Furthermore, even if separation of this type does not occur, the degree of expansion of the base 6, the nonmagnetic metal layer 7, and the magnetic layer 8 is different due to the difference in their coefficients of thermal expansion causing the base 6, the non-magnetic metal layer 7 and the magnetic layer 8 to be subjected to stress in correspondence with the degree of thermal expansion. As a result of this, the magnetic characteristics of the magnetic layer 8 deteriorate, causing the sensitivity of the magnetic scale itself to be lowered.
It is known that the conventional way of using a magnetic tape or the like as a magnetic scale involves demagnetizing the magnetic tape or the like when subjected to heat. Thus, conventionally magnetic tape or the like is used only at temperatures of, for example, less than 50.degree. C.